Deep brain stimulation of the subthalamic nucleus is an effective treatment for Parkinson's disease, although its precise mechanisms remain poorly understood. To gain further insight into the mechanisms underlying deep brain stimulation, we analysed the causal relationship between forearm muscle activity and local field potentials derived from the subthalamic nucleus. In 19 patients suffering from Parkinson's disease of the akinetic-rigid subtype, we calculated the squared partial directed coherence between muscles of the contralateral forearm and the subthalamic nucleus or zona incerta during both a rest and a hold condition of the arm. For both recording regions, data analysis revealed that, during the rest condition, electromyographic activity was significantly more often 'Granger-causal' for the local field potentials than the opposite causation. In contrast, during the hold condition, no significant difference was found in the occurrence of causalities. Contrary to the existing basal ganglia model and the current concept of Parkinson's disease pathophysiology, we found the subthalamic nucleus to receive more 'afferences' than it emitted 'efferences', suggesting that its role is more complex than a simple driving nucleus in the basal ganglia loop. Therefore, the effect of deep brain stimulation in the subthalamic nucleus could, at least in part, result from a blockade of pathological afferent input.